Protecting‐Group‐Free Total Synthesis of Isoquinoline Alkaloids by Nickel‐Catalyzed Annulation ofo‐Halobenzaldimine with an Alkyne as the Key Step

Abstract

An efficient short total synthesis of benzo[c]phenanthridine alkaloids including oxyavicine, oxynitidine, and oxysanguinarine is described. Thus, N-methyl-o-bromobenzaldimines 1 b-d undergo regioselective cyclization with 4-(benzo[d][1,3]dioxol-5-yl)but-3-yn-1-ol (2 b) in the presence of [Ni(cod)(2)] (cod=1,5-cyclooctadiene). In situ oxidation of the resultant isoquinolinium salts gives isoquinolinone derivatives 5 b-d with benzo[d][1,3]dioxol-5-yl substitution at the C(3) atom and a (CH(2))(2)OH group at the C(4) atom. Later, oxidation of the alcohol group in 5 b-d to the aldehyde moiety followed by acid-catalyzed cyclization and dehydration completes the total syntheses to give oxyavicine, oxynitidine, and oxysanguinarine in 67, 65, and 60 % yields, respectively. The synthesis requires four steps from o-bromobenzaldehyde derivatives. Transformations of these alkaloids to the other alkaloids in this family are also discussed herein.